1. Field of the Invention
The present invention relates to a waveguide filter used in a communication apparatus of microwave and millimeter wave band.
2. Description of the Prior Art
In satellite transmission, since a ground station and a satellite are separated by a distance as long as 35,900 km (in the case of a stationary satellite), radio wave inputted to a receiver is very weak. Consequently, a filter to treat such weak radio wave must be one with small passing transmission loss. An example of a filter with small loss adopted frequently is a wavequide filter having high selectivity Q. Also in the transmission side, since high power is transmitted, energy lost by the passing transmission is converted into heat energy thereby a transmitting apparatus may be heated in similar manner to the receiving side. Consequently, a waveguide filter with small passing transmission loss is frequently adopted not only in the receiving side but also in the transmission side.
In usual, a waveguide filter in basic structure as shown in FIG. 3 comprises a waveguide with square cross-section, and shunt inductor plates installed to the waveguide so as to divide it into a plurality of partitions each constituting a resonator. The waveguide filter is called a shunt inductor type waveguide filter.
In FIG. 3, numeral 21 designates a waveguide, numerals 22.about.29 shunt inductor plates to form induction windows (hereinafter referred to as "inductor plates"), and numerals 30.about.32 waveguide resonators. The inductor plates 22, 26, the inductor plates 23, 27, and part of the waveguide 21 enclosed by the inductor plates 22, 23, 26, 27 constitute one waveguide resonator 30, and the inductor plates 23, 24, 27, 28 and part of the waveguide 21 enclosed by these inductor plates constitute other waveguide resonator 31, and further the inductor plates 24, 25, 28, 29 and part of the waveguide 21 enclosed by these inductor plates constitute other waveguide resonator 32, thus these waveguide resonators 30.about.32 constitute a waveguide filter of three stages. The center frequency of resonance and the pass band width of the waveguide filter are determined by dimensions in width, height, length of the tube of the waveguide 21 and in width of each of the inductor plates 22.about.29 (size of the induction window).
Specific constitution of a waveguide filter in the prior art will be described referring to FIG. 4 and FIG. 5. In FIG. 4 and FIG. 5, parts similar to those in FIG. 3 are designated by the same reference numerals and the overlapped description shall be omitted.
In FIG. 4 and FIG. 5, numerals 33.about.40 designate grooves formed on the waveguide 21 for insertion of the induction plates 22.about.29, numerals 41, 42 flanges, and numerals 43.about.50 soldering by which the inductor plates 22.about.29 inserted in the grooves 33.about.40 are fixed to the waveguide 21. The grooves 33.about.40 of prescribed depth are formed at prescribed intervals on the waveguide 21, and the inductor plates 22.about.29 each being larger than height of the waveguide 21 or depth of the grooves 33.about.40 are inserted in the grooves 33.about.40 and then fixed to the waveguide 21 by soldering 43.about.50 from outside of the waveguide 21. In this case, in addition to the soldering, fixing means such as metal welding may be suitably selected. The flanges 41, 42 are installed on both ends of the waveguide 21 and used for connection to other waveguides or the like.
FIG. 6 is a sectional view taken in line A--A of FIG. 5. In FIG. 6, numerals 51.about.53 designates a transmission path of characteristic impedance Z determined by dimensions in width and height of the waveguide 21. FIG. 7 and FIG. 8 show equivalent circuits of FIG. 6. FIG. 7 is an equivalent circuit in ideal state where thickness of each of the inductor plates 22, 23, 26, 27 is zero. In actual state, however, the thickness of each of the inductor plates 22, 23, 26, 27 does not become zero but an equivalent circuit shown in FIG. 8 applies. That is, as shown in the circuit constitution of FIG. 8, the thickness of each of the inductor plates 22, 23, 26, 27 is represented by coils 54.about.57 inserted in series to the transmission path, and the coils 54.about.57 act to lower the center frequency of the pass band of the waveguide filter. Consequently, the inductor plates 22.about.29 used in the waveguide filter must be made as thin as possible. Numerals 58, 59 designate equivalent elements of the inductor plates 22, 23, 26, 27.
FIG. 9 is a fragmentary sectional view of the grooves 33.about.40 for insertion of the inductor plates 22.about.29. Top end portions of the inductor plates 22, 26 or bottom portions of the grooves 33, 37 cannot be made rectangular accurately on account of the maching technology, but in usual the top end portions of the inductor plates 22, 26 are cut slantwise or the bottom portions of the grooves 33, 37 are rounded.
FIG. 10 is a sectional view taken in line B--B of FIG. 5, and illustrates state that the inductor plates 22, 26 are mounted on the waveguide 21. The inductor plates 22, 26 are too thin to be pushed strongly against the grooves 33, 37 of the waveguide 21, thereby gaps 60.about.62 are apt to be produced between the top end portions of the inductor plates 22, 26 and the bottom portions of the grooves 33, 37 of the waveguide 21.
FIG. 11 is a sectional view taken in line C--C of FIG. 5, and illustrates state that the inductor plates 22.about.29 are mounted on the waveguide 21 in similar manner to FIG. 10. Since width of the grooves 37, 38 is not coincident with thickness of the inductor plates 26, 27, gaps 63.about.65 are apt to be produced between the inductor plates 26, 27 and the waveguide 21.
FIG. 12 illustrates state that the inductor plate 22 is fixed to the waveguide 21 by means of soldering. In usual, solder may flow out at inside of the waveguide 21 so as to produce a convex portion 66 on inside surface of the waveguide 21, or otherwise solder does not flow smoothly and cannot attain to the inside surface of the waveguide 21 so as to produce a concave portion 67 on the inside surface of the waveguide 21.
As above described, the waveguide filter in the prior art is apt to produce the gap, the convex portion or the concave portion on account of the machining technology, and such defect affects the dimension error of the waveguide filter and the surface current path, resulting in deviation of the center frequency and the pass band width of the waveguide filter.
For example, the dimension error will be described. In a waveguide filter of three-stage connection where the center frequency is 12 GHz, the pass band width is 200 MHz, the waveguide is 19.05 mm in width and 9.25 mm in height, and longitudinal distance between the inductor plates is 16.3.about.17.0 mm, the error of 0.1 mm in the longitudinal distance between the inductor plates results in variation of the center frequency by about 50 MHz, and also the error of 0.1 mm in dimension of the inductor plate inside the waveguide and width of the induction window results in variation of the pass band width by about 12 MHz.
In another waveguide filter of three-stage connection where the center frequency is 50 GHZ, the pass band width is 200 MHz, the waveguide is 4.78 mm in width and 2.39 mm in height, and the longitudinal distance between the inductor plates is 3.6.about.3.7 mm, the error of 0.01 mm in the longitudinal distance between the inductor plates results in variation of the center frequency by about 90 MHz, and also the error of 0.01 mm in dimensions of the inductor plate inside the waveguide and width of the induction window results in variation of the pass band width by about 10 MHz.
Thus the waveguide filter in the prior art has disadvantages in that the small dimension error results in large variation in the center frequency and the pass band width.
These factors will be mentioned specifically. As shown in FIG. 9, the top end portion of the inductor plate is slanted on account of the machining technology. The bottom portion of the groove for insertion of the inductor plate is rounded also on account of the machining technology. Consequently, even when the inductor plate is inserted in the groove, the gap is produced between the waveguide and the inductor plate as shown in FIG. 10. The inductor plate is too thin to be pushed strongly during the assembling thereby the gap cannot be eliminated. The gap may cause variation in dimensions of the inductor plate inside the waveguide and width of the induction window and deviation of the pass band width from a prescribed value.
Since thickness of the inductor plate is not coincident with width of the groove, a gap as shown in FIG. 11 may be produced. The gap causes error in the longitudinal distance between the inductor plates and deviation of the center frequency. Further the gap deteriorates the return loss and the ripple characteristics of the pass band.
If the convex portion or the concave portion due to soldering is produced as shown in FIG. 12, the surface current path is lengthened by the convex portion or the concave portion and the center frequency deviates from the prescribed value. Since the junction surface between the waveguide and the inductor plate is not smooth on account of the convex portion or the concave portion due to soldering, high-frequency resistance in the junction surface increases and Q of the circuit decreases and the transmission loss in the pass band increases.
When the repair is performed at finding the dimension error or the defect in the inductor plate and the waveguide or for the part changing and the part correcting, since the fixing is performed by soldering and the waveguide has the integral structure, the repair work is very troublesome and the waveguide filter is not suitable for the mass production.